1. Field of the Invention
The present invention relates to a strength-enhancing apparatus for a metal part for enhancing the surface strength of the metal part.
2. Description of the Related Art
In general, the gear repeatedly receives the load when it is used. Therefore, it is necessary to enhance the fatigue strength of the gear surface. For this purpose, the shot peening has been hitherto widely performed to give the compressive residual stress, for example, by allowing steel balls to make collision against the gear surface.
However, the shot peening is inconvenient in that the gear surface becomes rough, and the surface roughness is deteriorated, because the steel balls are used as the shot material in the shot peening. In view of this point, as disclosed in Japanese Patent Publication No. 5-21711, a strength-enhancing method for the metal surface is known, in which a metal formed product is subjected to surface hardening, followed by grinding for the metal surface, and then glass beads having a grain diameter of 0.2 mm to 0.6 mm are impelled or projected thereagainst. Accordingly, it is intended to prevent the metal surface from being rough so that the fatigue strength is improved.
However, the conventional technique described above involves the following problems. That is, the given compressive residual stress is lowered, and it is impossible to improve and increase the fatigue strength up to a desired value. Further, the directivity of the projected glass beads is poor. Therefore, the glass beads are scattered in various directions, and consequently the efficiency is extremely lowered.
The present applicant has suggested a strength-enhancing apparatus for a gear which makes it possible to give a sufficient compressive residual stress and obtain a smooth surface over an area ranging from the tooth surface to the tooth root. A patent application has been filed therefor (see Japanese Laid-Open Patent Publication No. 9-248761). In this prior art, there are provided a gear-holding mechanism for positioning and holding, in a chamber, a gear after being subjected to a heat treatment, an impelling mechanism for impelling or projecting a spouting stream of glass beads and liquid from a nozzle toward a gear surface, a liquid supply mechanism for supplying the liquid to the impelling mechanism under a pressure, and a glass bead supply mechanism for successively feeding a predetermined amount of the glass beads to the impelling mechanism. Accordingly, the glass beads correctly collide against the gear surface while maintaining the directivity. A desired compressive residual stress is given to the gear surface. Further, a smooth surface is obtained over an area ranging from the tooth surface to the tooth root of the gear surface as the glass beads are crushed.
The glass beads collide against the gear surface as the metal surface, and they are crushed. Therefore, the glass bead dust (hereinafter referred to as "powder flow dust" as well) in a micron order floats in the processing chamber. However, the gear, which is subjected to the treatment, is rotated at a high speed while being installed to a spindle. Therefore, the following problems occur. That is, the minute powder flow dust tends to adhere to the spindle rotating at the high speed. The spindle suffers an inconvenience such as rotation defect.
In view of the above, a structure is usually known and used, in which water is jetted or spouted toward the portion at which the powder flow dust causes adhesion and accumulation in the processing chamber so that the powder flow dust is removed therefrom. However, the mist containing the powder flow dust floats in the processing chamber. Such a structure fails to effectively remove the mist. Therefore, a problem is pointed out in that it is impossible to reliably dissolve the adhesion and accumulation of the powder flow dust.
Further, the powder flow dust as described above tends to leak from the processing chamber to the outside, because the dust is extremely minute. Various problems arise, for example, concerning the maintenance of the apparatus and the environment around the apparatus. Furthermore, a large noise is generated when the spouting stream of the glass beads and the liquid is projected onto the metal surface. A problem is also pointed out concerning the noise control.
In the gear-holding mechanism described above, the gear is installed to the spindle provided for the spindle unit. The gear is rotated integrally with the spindle. However, in the case of such a structure, it is feared that any deflection occurs in the gear during the rotation, for example, when a lengthy gear such as a counter shaft is used. Therefore, the following problem is pointed out. That is, it is impossible to correctly project the spouting stream of the glass beads and the liquid toward the gear surface, and it is difficult to apply the highly accurate strength-enhancing treatment to the gear.
On the other hand, the present applicant has suggested a strength-enhancing apparatus for a gear which makes it possible to give a sufficient compressive residual stress and obtain a smooth surface over an area ranging from the tooth surface to the tooth root, and which makes it possible to reliably remove the minute glass bead dust. A patent application has been filed therefor (see Japanese Laid-Open Patent Publication No. 9-248765).
In this prior art, there are provided an impelling mechanism for impelling or projecting, in a chamber, a spouting stream of glass beads and liquid from a nozzle toward a gear surface after being subjected to a heat treatment, and a recovery mechanism for sucking and recovering powder flow dust generated from the glass beads crushed on the gear surface. The recovery mechanism includes a suction port which faces the inside of the chamber and which is arranged in the vicinity of the gear. Accordingly, the glass beads correctly collide against the gear surface while maintaining the directivity. A desired compressive residual stress is given to the gear surface. Further, the minute powder flow dust, which is generated as the glass beads are crushed, is reliably sucked and recovered from the suction port.
The recovery mechanism described above is used such that the mist containing the powder flow dust floating in the chamber is sucked and discarded. However, when the strength-enhancing treatment is continuously performed for the gear, the amount of discarded drainage arrives at a considerable amount. For this reason, it is difficult to reliably remove the powder flow dust from the inside of the chamber. Further, the powder flow dust, which is contained in the drainage, can be used to produce the glass beads. On the other hand, the liquid can be recycled as the washing water to be used in the chamber.